Lithium ion secondary batteries are secondary batteries having a high charge/discharge capacity and capable of achieving high output. Currently, lithium ion secondary batteries are mainly used as power supplies for portable electronic equipment, and are expected to be used as power supplies for electric vehicles assumed to be used widely in the future. Lithium ion secondary batteries have, respectively in a positive electrode and a negative electrode, active materials capable of inserting and eliminating lithium (Li) therein/therefrom. The lithium ion secondary batteries operate when lithium ions move through an electrolytic solution provided between the two electrodes.
In lithium ion secondary batteries, a lithium-containing metallic complex oxide such as a lithium cobalt complex oxide is mainly used as the active material for the positive electrode, and a carbon material having a multilayer structure is mainly used as the active material for the negative electrode. The performance of a lithium ion secondary battery is influenced by materials of the positive electrode, the negative electrode, and the electrolyte that are included in the secondary battery. Research and development are actively conducted for active material substances forming the active materials. For example, usage of silicon or a silicon oxide having a higher capacity than carbon is discussed as a substance for the negative electrode active material.
When silicon is used as the negative electrode active material, a battery with a capacity higher than when a carbon material is used is obtained. However, silicon undergoes a large volume change associated with occlusion and release of Li during charging and discharging. Thus, in a secondary battery in which silicon is used as a negative electrode active material, silicon turns into fine powder associated with a volume change thereof during charging and discharging, so that silicon becomes eliminated or detached from a current collector. Therefore, this secondary battery has a problem of short charge/discharge cycle life of the battery. For that reason, a technique to suppress a volume change associated with occlusion and release of Li during charging and discharging by using a silicon oxide as a negative electrode active material, as compared to silicon, is discussed.
For example, usage of a silicon oxide (SiOx: x is about 0.5≤x≤1.5) is discussed as the negative electrode active material. SiOx, when being heated, is known to decompose into Si and SiO2. This is referred to as a disproportionation reaction in which a solid separates into two phases, i.e., Si phase and SiO2 phase, through an internal reaction. The Si phase obtained from the separation is extremely fine. In addition, the SiO2 phase that covers the Si phase has a function of suppressing decomposition of the electrolytic solution. Thus, the secondary battery using the negative electrode active material formed of SiOx that has been decomposed into Si and SiO2 has excellent cycle characteristics.
The cycle characteristics of the secondary battery improve further when finer silicon particles forming the Si phase of the SiOx described above are used as a negative electrode active material in the secondary battery. JP3865033 (B2) (Patent Literature 1) discloses a method of heating metal silicon and SiO2 to sublimate those into a silicon oxide gas, and cooling the gas to produce SiOx.
JP2009102219 (A) (Patent Literature 2) discloses a production method including decomposing a silicon raw material into an elemental state in a high temperature plasma, rapidly cooling it to the temperature of liquid nitrogen to obtain silicon nano particles, and fixing the silicon nano particles into a SiO2—TiO2 matrix by using a sol-gel method or the like.
In the production method disclosed in Patent Literature 1, the materials are limited to sublimable materials. Moreover, irreversible Li is known to be generated at the negative electrode due to change of the SiO2 phase, which covers the Si phase, into lithium silicate at the time of Li occlusion, and thus it is necessary to add an extra active material to the positive electrode. In addition, in the production method disclosed in Patent Literature 2, high energy is required for plasma discharge. Furthermore, in silicon based materials obtained from these production methods, the silicon particles of the Si phase are speculated to have low dispersibility and be easily aggregated. When the silicon particles aggregate with each other and the particle sizes thereof become large, the secondary battery using those as the negative electrode active material results in having a low initial capacity and deteriorated cycle characteristics.
In recent years, nano silicon materials that are expected for usage in semiconductors, electrics or electronics fields, and the like have been developed. For example, Physical Review B (1993), vol. 48, pp. 8172-8189 (Non-Patent Literature 1) discloses a method for synthesizing a layered polysilane by causing a reaction between hydrogen chloride (HCl) and calcium disilicide (CaSi2), and states that the layered polysilane obtained in this manner can be used in a light-emitting element or the like.
Materials Research Bulletin, Vol. 31, No. 3, pp. 307-316, 1996 (Non-Patent Literature 2) states that plate-like silicon crystal was obtained by performing a heat treatment at 900° C. on a layered polysilane obtained by causing a reaction between hydrogen chloride (HCl) and calcium disilicide (CaSi2).
JP2011090806 (A) (Patent Literature 3) discloses a lithium ion secondary battery in which a layered polysilane is used as a negative electrode active material.